Mac-Compatible USB Wireless Adapters?

Out of curiosity, are there any USB wireless adapters that work with Mac OS X? I work at a CompUSA and there are sometimes customers that have older Macs that use the Airport cards that are no longer available. What would be a good USB alternative for them?

I've been told (on this forum) there aren't any that work reliably on a Mac. A search on eBay shows a few that claim to work on a Mac. It also reveals that the older Airport cards (802.11b) are going for around the $100 mark.

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